1. Field of the Invention
The present invention relates generally to a system for minimizing the deleterious impact of oil-derived phosphorus containing compounds on automotive exhaust aftertreatment systems.
2. Background of the Invention
Automotive oils typically contain a zinc dialkyldithiophosphate (ZDDP) additive which forms an antiwear coating on engine components and acts as an antioxidant in the oil. Although engines are designed to minimize the quantity of engine oil exiting the engine via the combustion chamber and exhaust system, it is inevitable that a small fraction of engine oil is released by this mechanism. The ZDDP additive of engine oil deleteriously affects catalytic converters due to phosphorus from the ZDDP interfering with active sites within the catalyst. These phosphorus containing species deposit onto, or react with washcoat components, such as aluminum oxide and cerium oxide, and remain there indefinitely. This phenomenon is commonly referred to as phosphorus poisoning.
Measures to eliminate or reduce ZDDP in engine oils have been investigated. Alternatives to ZDDP have been produced which have been shown to provide antioxidant and antiwear properties similar to ZDDP. However, they are cost prohibitive. Engine oils may be formulated with a lesser amount of ZDDP with the consequences that engine wear and oil oxidation increase, the former limiting engine life and the latter reducing useful oil life.
The inventor of U.S. Pat. No. 5,857,326 has disclosed an exhaust poison trap which comprises a helical wall dividing the exhaust chamber into longitudinal helical passages for exhaust gas flow and porous means covering the interior of the peripheral wall. The inventor of '326 teaches that exhaust gas is directed in a helical path causing particulate matter in the gas to be accelerated outwardly by centrifugal force and trapped in the porous means. The inventor of the present invention has recognized several limitations of the approach in '326. The helical passages cause the exhaust gases to be rotated and particles that have a diameter less than a certain size follow the flow and avoid being trapped in the porous means near the walls of the tube and larger particles impact the porous means near the walls. The device disclosed in '326 has the capability of causing only the largest particles to be removed. The figures in '326 indicate that the helical wall causes the flow to rotate through at least two revolutions and as many as four revolutions. The length of the exhaust poison trap is approximately two to four pipe diameters long with the disadvantages of complicating the packaging of the exhaust poison trap and increasing the weight of the trap, the thermal mass of which interferes with the desire to bring the catalytic converter to its operating temperature as soon as possible after starting the engine to control cold start emissions.